Method of making a reversible matrix drug delivery system

ABSTRACT

A method for producing a drug system which is a reversible gelled matrix with entrapped drug. The gel is made by combining a polysaccharide, such as dextran, a polymer of glucose, with a natural macromolecule, such as the lectin, concanavalin A, with binding sites for monomers of the polysaccharide. The drug is released in response to exposure of the matrix to free carbohydrate containing the monomer of the polysaccharide for which the natural macromolecule binds.

This application is a continuation of U.S. Ser. No. 08/256,283, filed onAug. 11, 1994 (now U.S. Pat. No. 5,830,506), which is the national phaseapplication of PCT Application Number PCT/GB93/00034 filed Jan. 8, 1993.

This invention concerns a method for producing a drug system fordelivering a drug or other agent for the treatment of a condition in acontrolled release manner.

Most known ‘controlled-release’ drug systems are constant slow-releasemechanisms (U.S. Pat. No. 4,145,410). These systems do not react to anyinherent substances in the body to release the drug appropriately inresponse to that substance, ie. there is no feedback mechanism. Feedbackmechanisms have been suggested to be of interest in vivo to maintaingeneral homeostasis.

U.S. Pat. No. 4,348,387 discloses a feedback controlled insulin deliverysystem wherein glucose-insulin conjugates are displaced from glucosebinding sites on a binding molecule by free glucose. The conjugatedinsulin retains its biological activity once released. However, it isnot known whether other agents or drugs can be so conjugated or whethersuch a conjugated form of any drug or other agent would be effective.

The present invention provides a controlled drug delivery system for thecontrolled release of an unconjugated drug or other agent.

According to the present invention there is provided a method forproducing a drug system for delivering a drug for the treatment of acondition, comprising immobilising a drug or other agent in abio-compatible matrix containing at least one receptor for aphysiological substance which will be in the environment of the matrixwhen administered, said receptor being activated in response to thelevels of said physiological substance to effect a conformational changein the matrix allowing mobilisation and release of the drug into theenvironment.

The agent may comprise a naturally occurring biological agent, forexample, a hormone, which may, of course, be insulin, as in U.S. Pat.No. 4,348,387, but now in unconjugated form. Because there is now noneed for conjugation, other hormones or drugs which it may not bepossible to conjugate or which will not work in the system of U.S. Pat.No. 4,348,387 even if they can be conjugated, or which may bephysiologically ineffective or less effective or even harmful whenconjugated, may be used according to the invention.

The receptor may be a binding macromolecule, for example, a lectin,which may be concanavalin A, a lectin produced from the jack bean.

The receptor may reversibly bind the physiological substance, which maybe a carbohydrate, preferably glucose, or a carbohydrate containingglucose moieties, such as dextran.

Insulin, or any other agent or drug, may be immobilised in the matrixwhen terminal glucose molecules on dextran bind to concanavalin A toform a gel.

The conformational change in the matrix may be brought about by thedisplacement of terminal dextran-glucose molecules from the receptor byfree glucose in the physiological environment which may of course beblood or other tissue fluid.

The conformational change may be an ungelling of the matrix allowingmobilisation and release of the insulin or other agent or drug.

The invention will be further apparent from the following descriptionwith reference to the several figures of the accompanying drawings,which show, by way of example only, one form of the system embodyingsame.

Of the drawings:

FIG. 1 shows a diagrammatical representation of a concanavalin-Amolecule binding glucose;

FIG. 2 shows a diagramatical representation of the delivery system, inan inactive form;

FIG. 3 shows a diagramatical representation of the delivery system, inan active form;

FIG. 4 shows the drug system of FIG. 2 enclosed in a bio-compatiblelattice; and

FIGS. 5 to 7 show in vitro experimental results of the drug system.

Referring now to the drawings, it will be seen that the drug deliverysystem is based on a concanavalin-A molecule 11. Each concanavalin-Amolecule 11 has four binding sites 12 specific for sugars, with a highaffinity for glucose 13 (FIG. 1). The binding of glucose 13 to thebinding site 12 is, however, reversible.

The inactive form of the drug system comprises (FIG. 2) glucose presentas terminal glucose moieties 15 of the carbohydrate dextran. Dextran isa branched chain polysaccharide with many terminal glucose moietieswhich can bind to the binding sites 12 of concanavalin-A molecules 11and in so doing cross-link the concanavalin-A molecules together to forma viscous gel matrix. Insulin 16 is premixed with dextran gel, so thatwhen concanavalin-A is added to the mixture, a gel is formed between thedextran and the concanavalin-A with the insulin relatively immobilisedinside the gel matrix unable to escape rapidly (FIG. 2).

The binding of dextran to concanavalin-A is reversed (FIG. 3) when thereis an increase in the concentration of free glucose 17 in thephysiological environment. The free glucose displaces the terminaldextran-glucose 15 from the binding sites of the concanavalin Amolecules. The matrix undergoes a conformational change allowingmobilisation and release of the insulin into the environment.

Once the level of free glucose falls, in response to the action of thereleased insulin, the displaced terminal dextran-glucose molecules willre-bind to the concanavalin-A molecules and the matrix will re-gel, thusagain restricting insulin remaining in the matrix.

The mechanism of this drug system is thus repeatable and releasesinsulin in response to a number of free glucose insults, in a similarmanner to the in vivo feedback mechanism of the pancreatic cells.

Experiments carried out in vitro show release of a drug, nitrofurantoin,from the drug system in response to a glucose load. The drugnitrofuantoin was chosen to exemplify the invention mainly because ithas similar physicochemical properties to some oral hypoglyceamic drugsbut is easier to assay. In these experiments the drug system comprised amixture of the following:

Dextran (from Sephadex G-25, fine beads)  99 mg Drug (Nitrofurantoin) 39 mg Concanavalin-A preparation comprising sodium chloride  15 mglectin  3 mg Ethocel 50 cp (water dispersible) 153 mg TOTAL 300 mg

The drug system above was separately tested in both sucrose containingand plain media. The sugar was present at 2% w/v. The pH was 6.2 and themedia contained trace quantities of manganese and calcium, important forglucose moiety binding to lectin.

Dextran (uncross-linked AW 2-40×10⁶ daltons) may be used in place ofsephadex G-25 fine beads.

The results are shown in FIG. 5. As can be seen from the graph, the drugwas released more quickly from the lectin in the presence of sucrose. Afurther control experiment was carried out in which the drug system wasmixed without any lectin. The results are shown in FIG. 6. Areproducible depression of drug release was seen in the presence ofsucrose, probably due to to the viscosity of the diffusion boundarylayers. When a correction was made for this effect in the experiment ofFIG. 5, the increased rate of release of the drug was even morepronounced (FIG. 7).

The system could be implanted inside a diabetic patient with up toseveral months supply of insulin. However, since concanavalin-A isitself toxic, it would be desirable to enclose the matrix in anon-erodible material which allows the diffusion of free glucose andinsulin into and out of the matrix, but does not allow the escape of theconcanavalin-A molecules.

An example of such a material is an acrylic permanent gel, covalentlycross-linked in situ to form a three dimensional grid structure 18 (FIG.4). In addition to preventing the escape of toxic concanavalin-A intothe bloodstream of a patient, it also allows an implant to berecoverable and possibly ‘recharged’ with insulin and reused. Theacrylic network will also help to prevent overload of the receptor by ahigh level of irrelevent sugar with subsequent loss of specificity aswell as preventing dose-dumping. In such a matrix, restriction of drugdiffusion may occur by either an increase in viscosity or precipitationvia the dextran-lectin reaction.

Alternatively, the dextran perimeter may be cross-linked with a specificagent, for example, epichlorohydrin to form a membrane of cross-linkeddextran around the gel.

It will be appreciated that it is not intended to limit the invention tothe above example only, many variations, such as might readily occur toone skilled in the art, being possible, without departing from the scopethereof.

For example, when adminstering anything other than insulin or a drugwhich is intended to control blood sugar, the drug or agent may bereleased not by a feed-back mechanism but by a controlled administrationof sugar, bringing literally to life the notion of a spoonful of sugarmaking the medicine go down.

The receptor may be any molecule, natural or engineered, having bindingsites for complex branched substrates which would form a gel on binding,but would be displaceable by the ‘free’ substrate which may be naturallyor artificially induced to effect release of a drug or other agent.Lectins other than concanavalin-A, which may be non-toxic or less toxic,may be used, for example, the pea (Pisium sativum) lectin.

1. A method of producing a system for delivering a drug or otherbiological agents, comprising the step of combining a reversible gelledmatrix and the drug or other biological agents, wherein the reversiblegelled matrix comprises a crosslinking agent and at least one receptorhaving a plurality of binding sites for a substrate in the environmentof the matrix and for the crosslinking agent, the at least one receptorbeing activated in response to the levels of the substrate to effect aconformational change in the matrix allowing mobilisation and release ofthe drug or other biological agents into the environment, theconformational change in the matrix allowing mobilisation being effectedby the binding of the substrate to the binding sites such that itdisplaces the crosslinking agent from the binding sites, and theconformational change in the matrix allowing immobilisation beingeffected by the binding of the crosslinking agent to the binding sitessuch that when the crosslinking agent becomes bound to the binding sitesit displaces the substrate from the binding sites.
 2. A method accordingto claim 1, wherein the drug comprises a naturally occurring biologicalagent.
 3. A method according to claim 2, wherein the biological agent isa hormone.
 4. A method according to claim 3, wherein the hormone isinsulin.
 5. A method according to claim 1, wherein the receptor is abinding macromolecule.
 6. The method according to claim 1, wherein theat least one receptor is a lectin.
 7. A method according to claim 6,wherein the lectin is concanavalin A, a jack bean lectin.
 8. A methodaccording to claim 6, wherein the lectin is a pea lectin.
 9. The methodaccording to claim 1, wherein the at least one receptor is synthetic.10. A method according to claim 1, wherein the substrate is aphysiological substance.
 11. A method according to claim 10, wherein thephysiological substance is a carbohydrate.
 12. A method according toclaim 11, wherein the carbohydrate is glucose.
 13. The method accordingto claim 12, wherein the crosslinking agent is dextran, terminal glucosemoieties on the dextran binding to the at least one receptor.
 14. Themethod according to claim 1, wherein the drug is immobilised in thegelled matrix when terminal glucose moieties on dextran bind toconcanavalin A.
 15. The method according to claim 1, wherein theconformational change in the matrix allowing mobilisation is anungelling and the conformational change in the matrix allowingimmobilisation is a gelling.
 16. The method according to claim 1,wherein the system delivers a drug or other biological agents for thetreatment of a condition.
 17. A system for delivering a drug or otherbiological agents, comprising a reversible gelled matrix containing animmobilised drug or other biological agents, wherein the reversiblegelled matrix comprises a crosslinking agent and at least one receptorhaving a plurality of binding sites for a substrate in the environmentof the matrix and for the crosslinking agent, the at least one receptorbeing activated in response to the levels of the substrate to effect aconformational change in the matrix allowing mobilisation and release ofthe drug or other biological agents into the environment, theconformational change in the matrix allowing mobilisation being effectedby the binding of the substrate to the binding sites such that itdisplaces the crosslinking agent from the binding sites, and theconformational change in the matrix allowing immobilisation beingeffected by the binding of the crosslinking agent to the binding sitessuch that when the crosslinking agent becomes bound to the binding sitesit displaces the substrate from the binding sites.
 18. The systemaccording to claim 17, wherein the conformational change in the matrixallowing mobilisation is an ungelling and the conformational change inthe matrix allowing immobilisation is a gelling.
 19. The systemaccording to claim 17, wherein the system delivers a drug or otherbiological agents for the treatment of a condition.
 20. Te method ofclaim 1, wherein the at least one receptor does not have a binding sitefor the drug or biological agents.
 21. The method of claim 17, whereinthe at least one receptor does not have a binding site for the drug orbiological agents.
 22. A method of producing a system for delivering adrug or other biological agents, comprising the step of combining: adrug or other biological agents; at least one polyfunctional branchedpolysaccharide crosslinking agent; and at least one polyfunctionalreceptor, wherein the at least one crosslinking agent and the at leastone receptor form a reversible gelled matrix and the at least onereceptor has a plurality of binding sites for a substrate in theenvironment of the matrix and for the crosslinking agent.
 23. The methodaccording to claim 22, wherein the at least one receptor is activated inresponse to the levels of the substrate to effect a conformationalchange in the matrix allowing mobilisation and release of the drug orother biological agents into the environment, a conformational change inthe matrix allowing mobilisation is effected by the binding of thesubstrate to the binding sites such that it displaces the crosslinkingagent from the binding sites, and the conformational change in thematrix allowing immobilisation is effected by the binding of thecrosslinking agent to the binding sites such that when the crosslinkingagent becomes bound to the binding sites it displaces the substrate fromthe binding sites.
 24. The method according to claim 22, wherein the atleast one polyfunctional crosslinking agent is dextran.
 25. The methodaccording to claim 22, wherein the substrate is a physiologicalsubstance.
 26. The method according to claim 25, wherein thephysiological substance is a carbohydrate.
 27. A method of producing asystem for delivering a drug or other biological agents, comprising thestep of combining a reversible gelled matrix and the drug or otherbiological agents, wherein the reversible gelled matrix comprises acrosslinking agent and at least one receptor having a plurality ofbinding sites for a substrate in the environment of the matrix and forthe crosslinking agent while not having binding sites for the drug orother biological agents, the at least one receptor being activated inresponse to the levels of the substrate to effect a conformationalchange in the matrix allowing mobilisation and release of the drug orother biological agents into the environment, the conformational changein the matrix allowing mobilisation being effected by the binding of thesubstrate to the binding sites such that it displaces the crosslinkingagent from the binding sites, and the conformational change in thematrix allowing immobilisation being effected by the binding of thecrosslinking agent to the binding sites such that when the crosslinkingagent becomes bound to the binding sites it displaces the substrate fromthe binding sites.
 28. A system for delivering a drug or otherbiological agents, comprising a reversible gelled matrix containing animmobilised drug or other biological agents, wherein the reversiblegelled matrix comprises a crosslinking agent and at least one receptorhaving a plurality of binding sites for a substrate in the environmentof the matrix and for the crosslinking agent while not having bindingsites for the drug or other biological agents, the at least one receptorbeing activated in response to the levels of the substrate to effect aconformational change in the matrix allowing mobilisation and release ofthe drug or other biological agents into the environment, theconformational change in the matrix allowing mobilisation being effectedby the binding of the substrate to the binding sites such that itdisplaces the crosslinking agent from the binding sites, and theconformational change in the matrix allowing immobilisation beingeffected by the binding of the crosslinking agent to the binding sitessuch that when the crosslinking agent becomes bound to the binding sitesit displaces the substrate from the binding sites.
 29. The systemaccording to claim 28, wherein the conformational change in the matrixallowing mobilisation is an ungelling and the conformational change inthe matrix allowing immobilisation is a gelling.
 30. A method ofproducing a system for delivering a drug or other biological agents,comprising the step of combining: a drug or other biological agents; atleast one polyfunctional branched polysaccharide crosslinking agent; andat least one polyfunctional receptor, wherein the at least onecrosslinking agent and the at least one receptor form a reversiblegelled matrix and the at least one receptor has a plurality of bindingsites for a substrate in the environment of the matrix and for thecrosslinking agent while not having binding sites for the drug orbiological agents.
 31. The method according to claim 30, wherein the atleast one receptor is activated in response to the levels of thesubstrate to effect a conformational change in the matrix allowingmobilisation and release of the drug or other biological agents into theenvironment, a conformational change in the matrix allowing mobilisationis effected by the binding of the substrate to the binding sites suchthat it displaces the crosslinking agent from the binding sites, and theconformational change in the matrix allowing immobilisation is effectedby the binding of the crosslinking agent to the binding sites such thatwhen the crosslinking agent becomes bound to the binding sites itdisplaces the substrate from the binding sites.
 32. A method ofproducing a system for delivering a drug or other biological agents,comprising the step of combining a reversible gelled matrix and the drugor other biological agents, wherein the reversible gelled matrixcomprises a branched polysaccharide crosslinking agent and at least onelectin receptor having a plurality of binding sites for a substrate inthe environment of the matrix and for the crosslinking agent, the atleast one receptor being activated in response to the levels of thesubstrate to effect a conformational change in the matrix allowingmobilisation and release of the drug or other biological agents into theenvironment the conformational change in the matrix allowingmobilisation being effected by the binding of the substrate to thebinding site such that it displaces the crosslinking agent from thebinding sites, and the conformational change in the matrix allowingimmobilisation being effected by the binding of the crosslinking agentto the binding sites such that when the crosslinking agent becomes boundto the binding sites it displaces the substrate from the binding sites.33. The method of claim 1 wherein the drug or biological agentsimmobilised in the matrix is unconjugated.
 34. The method of claim 17wherein the drug or biological agents immobilised in the matrix isunconjugated.
 35. The method of claim 22 wherein the drug or biologicalagents immobilised in the matrix is unconjugated.
 36. The method ofclaim 27 wherein the drug or biological agents immobilised in the matrixis unconjugated.
 37. The method of claim 28 wherein the drug orbiological agents immobilised in the matrix is unconjugated.
 38. Themethod of claim 30 wherein the drug or biological agents immobilised inthe matrix is unconjugated.
 39. The method of claim 32 wherein the drugor biological agents immobilised in the matrix is unconjugated.